Auto merge of #23095 - jdm:mozjsprofiling, r=jdm

Use mozjs profiling feature when building with frame pointer enabled

This makes it possible to get more meaningful profiles with #23080.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix part of #23081
- [x] There are no tests for these changes

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python/servo/build_commands.py

@@ -283,6 +283,7 @@ class MachCommands(CommandBase):
 
             opts += ["--target", target]
 
+        env = self.build_env(target=target, is_build=True)
         self.ensure_bootstrapped(target=target)
         self.ensure_clobbered()
 
@@ -291,6 +292,10 @@ class MachCommands(CommandBase):
         if debug_mozjs:
             features += ["debugmozjs"]
 
+        if with_frame_pointer:
+            env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -C force-frame-pointers=yes"
+            features += ["profilemozjs"]
+
         if self.config["build"]["webgl-backtrace"]:
             features += ["webgl-backtrace"]
         if self.config["build"]["dom-backtrace"]:
@@ -300,15 +305,11 @@ class MachCommands(CommandBase):
             opts += ["--features", "%s" % ' '.join(features)]
 
         build_start = time()
-        env = self.build_env(target=target, is_build=True)
         env["CARGO_TARGET_DIR"] = target_path
 
         if with_debug_assertions:
             env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -C debug_assertions"
 
-        if with_frame_pointer:
-            env['RUSTFLAGS'] = env.get('RUSTFLAGS', "") + " -C force-frame-pointers=yes"
-
         if android:
             if "ANDROID_NDK" not in env:
                 print("Please set the ANDROID_NDK environment variable.")